Data signal detectors of the prior art have utilized a technique which accumulates the number of transitions in the data signal occurring in quadrature with the clock signal in order to detect the presence of noise. Thus, noise is present when data transitions occur in quadrature with the clock signal, and data is present when data transitions occur synchronously with the clock signal. A system of this type is described in U.S. Pat. No. 3,995,225. However, the foregoing data detection system requires recovery of a clock signal with the proper phase relationship to the data signal and has relatively slow response times.
According to another technique, the time between successive zero crossings of the recovered data signal is used to detect the presence of a PSK data signal. When the PSK data signal is present, the time between successive zero crossings will be one of two possible time intervals. A system using this technique to detect the presence of a PSK data signal is described in U.S. Pat. No. 3,939,431.
According to a further technique for detection of the presence of a data signal, the frequency spectrum of the received data exhibits predictable characteristics and may be selectively filtered to ascertain the presence or absence of the data signal. Such a data detector is described in U.S. Pat. No. 3,758,860 and in U.S. Pat. No. 4,156,867 entitled "Data Communication System", filed Sept. 6, 1977 by Stephen M. Bench, et al and assigned to the instant assignee. However, the foregoing data detectors have a relatively wide bandwidth and relatively slow response times.